The invention relates to an optical unit having elements which lie one next to the other without an air gap therebetween. This joining technique is characterized as xe2x80x9cwringingxe2x80x9d or as xe2x80x9cphysical gluexe2x80x9d. At least one element of the optical unit is made of crystalline material. The invention also relates to an optical system incorporating the optical unit. The invention further relates to methods of making the optical unit.
Wringing is widely used in optical manufacture as an alternative to cementing. However, as a joining technique for crystal, wringing is effective only to a limited extent especially where the elements of the unit have arcuate surfaces. It is at these surfaces, that microscopic steps are formed at the transitions between crystalline planes. A generally low adhesion occurs where the glass elements are made of fluorides.
Crystals and especially fluorides, such as CaF2, MgF2 and NaF, are required for ultraviolet optics such as for the deep ultraviolet range below a wavelength of 250 nm. These crystals are needed because they belong to those materials which are transparent in this wavelength range and which are photochemically stable and they can supplement quartz glass with their other optical characteristics such as for achromatization. Double refracting characteristics are also of significance.
Cementing cannot be used as a joining technique for this wavelength range because all these organic adhesive substances are not resistant to ultraviolet radiation.
It is an object of the invention to provide a technique for joining crystalline material without an intermediate air gap and without cement and to also provide optical units, systems and methods of manufacture with this technique.
The optical unit of the invention includes: a first optical element made of crystalline material; a second optical element; the first and second optical elements being mutually adjacent and conjointly defining an interface therebetween devoid of an air gap; the first optical element having a surface facing toward the second optical element at the interface; and, the first optical element having an amorphous inorganic layer formed on the surface in the interface.
According to a feature of the invention, the crystalline material is a fluoride. This is especially advantageous because, with this material, wringing is especially difficult and weak.
According to another feature of the invention, the amorphous inorganic layer is made of quartz glass and this material is suitable, inter alia, because of its ultraviolet transparency and resistance to ultraviolet light.
In this way, an achromatic unit suitable for deep ultraviolet light can be assembled using quartz glass and calcium fluoride with the optical elements being wrung to each other. The two optical elements can be made of different materials. The optical unit of the invention can be used with an ultraviolet light source and especially with a deep ultraviolet light source.
In an embodiment of the method of the invention, the element made of crystalline material such as fluoride is prepared for wringing. This is done by applying an inorganic amorphous layer to the wringing surface. The inorganic amorphous layer can, for example, be made of quartz glass. The method of applying the layer is not important as numerous methods are suitable for this purpose. Excellent adhesion and a smooth surface results, for example, from sputtering.
As may be required, the surface of the layer made of crystalline material can be smoothed, for example, by polishing, tempering, applying ion radiation or chemical polishing.
The method of the invention is for making a thin optical element of crystalline material having a predetermined diameter and a thickness of less than 5 mm or less than 5% of the diameter. The thin optical element is formed from an unfinished piece made of the crystalline material and the unfinished piece has first and second surfaces lying opposite each other. The method includes the steps of: placing the unfinished piece of the crystalline material on a support; optically machining the first surface of the unfinished piece to the final form thereof; applying an amorphous inorganic layer to the first surface to define a coated first surface; wringing the unfinished piece to a lens holder of a polishing machine with the coated first surface being in adhering wrung contact engagement with the holder; and, optically machining the second surface to the final form thereof.
In the above method embodiment, the production of optical parts made of crystal, which, in use, do not remain wrung to another part, also affords advantages.
These advantages include that a partially processed piece made of crystal must not be cemented to a lens carrier in order to polish an opposite-lying end. Instead, the partially-processed piece can be wrung to another element after applying the coating according to a feature of the invention. Cement is therefore unnecessary. In lieu of being wrung to a lens carrier, especially thin elements can be wrung directly to a further optical component to which they remain connected during use.